Field of the Technology
The disclosure relates to the field of medical device, multimodality intravascular imaging systems, multimodality endoscopic imaging systems, optical coherence elastography, and acoustic radiation force systems.
Description of the Prior Art
Atherosclerosis is a complex disease in which multiple plaques build up within the arteries. The major cause of cardiovascular death in heart attacks (86%) and from brain aneurysm (45%) are due to less obtrusive plaques known as “vulnerable plaques” that rupture suddenly and trigger a blood clot or thrombus that blocks blood flow. Early detection of plaque lesions is the first and necessary step in preventing the lethal consequences of atherosclerosis. Diagnosis of the latent vulnerability of a plaque lesion relies on both tissue structural and biomechanical properties. The thickness of the fibrous cap, the thickness of the full plaques, and the vascular tissue biomechanical properties are all parameters that correlate with the vulnerability of the lesion. Although currently no clinical trials have confirmed the successful treatment of vulnerable plaque and reduction in cardiovascular mortality and morbidity, advances in clinical management of atherosclerosis require refinements to current therapies or new strategies with strict monitoring of all aspects of this epidemic disease.
Cancer is a leading worldwide cause of death. Early diagnosis of cancer increases the survival rate and reduces treatment. A cancerous tumor is normally stiffer than the background of normal soft tissue. For example, prostate cancer is the second most common cancer among American men, and is, behind lung cancer, the second most common cause of cancer deaths among men. Current diagnostic techniques, including digital rectal exam (DRE) and the measurement of blood prostate-specific antigen (PSA) levels, are insufficient for guiding treatment. Tissue biopsies are needed for diagnosis. The chief prostate cancer imaging technique is ultrasound, but grayscale ultrasound imaging is only 50-60% accurate and transrectal ultrasound (TRUS) is even less accurate. Ultrasound also has difficulty differentiating cancer from other diseases, such as benign prostatic hyperplasia (BPH) and prostatitis. Despite advances in ultrasound techniques such as color and power Doppler, and the introduction of ultrasound contrast agents, TRUS is still limited in accuracy and therefore only used to guide biopsy.
Currently, many biomedical imaging techniques aimed at imaging and assessing vulnerable plaques have been reported in the literature, including angiography, magnetic resonance imaging (MRI), intravascular ultrasound (IVUS), optical coherence tomography (OCT), intravascular near infrared spectral imaging, intravascular fluorescence imaging, and intravascular photoacoustic tomography (IVPAT).
Angioscopy allows the plaque to be visualized with high sensitivity, but the morphologic characterization of the plaque is unreliable because of the lack of estimation of cap thickness or lipid content. Even though MRI is used to study the progression and regression of plaque over time, its insufficient resolution cannot render accurate measurements.
IVUS has limited resolution and sensitivity to assess the thickness of the thin fibrous cap and for plaque classifications. OCT has a limited imaging depth and cannot quantify the full thickness of the plaque. Recently, Sawada et al. studied the feasibility of the combined use of IVUS and OCT data (images acquired separately) for detecting thin-cap fibroatheroma (TCFA) from 56 patients. The results clearly show that neither modality alone is sufficient for detecting TCFA.
The limitation of NIRS, Raman and fluorescence imaging is that they lack the capability of tomographic imaging. IVUS lacks the required high resolution to quantify the thickness of the fibrous cap. We recently reported the integrated OCT/US system for intravascular imaging. However, this system lacks the capability to resolve tissue composition in plaque lesions. Although each optical probing technique provides access to relevant diagnostic parameters, integration of several modalities is necessary to gather the information required to establish a robust method for early detection of plaque.
Elastography is an imaging technique, which measures the local deformation of tissue induced by stresses to estimate the strain and stiffness which are directly related to the biomechanical properties. The mechanical properties of tissue give important diagnostic information about many vascular diseases such as hypertension and coronary atherosclerosis. Due to the small dimensions involved, time varying blood flow, and presence of multiple plaques of different sizes, geometries, and compositions, arterial elastography is more challenging than other elastographies such as breast tissue elastography. Recently several elastography techniques have utilized other biomedical imaging modalities such as ultrasound, MRI, and OCT to detect the plaques in coronary arteries. For all elastography techniques, the resolution is determined by the underlying imaging modality.
Optical coherence elastography (OCE) has a superior micrometer scale resolution and is therefore suitable for imaging subtle mechanical changes in the early stages of disease. Excitation and detection are generally two characteristics or components of an OCE system. There are two main categories of OCE with respect to the excitation method: static/quasi-static excitation OCE and dynamic excitation OCE. The former applies compression to the subject statically/quasi-statically and measures the relaxation dynamics, which has limited imaging speed for in vivo imaging. The latter dynamically excites the subject using various waveforms and detects the induced displacement in a gated time window, which enables a much higher imaging speed and makes it possible for in vivo imaging. For the detection system, a phase-resolved OCT system can detect displacement with nanometer sensitivity, which is especially important for intravascular imaging as better sensitivity in displacement measurement means minimal force is needed to be applied to a vessel wall to quantify tissue mechanical properties. Phase-resolved dynamic OCE had been investigated previously for shear wave propagation detection, but this method requires separation between excitation and detection points, which limits the speed and spatial resolution. Adaptation of this method for in vivo real time 2D and 3D vascular elastography is not viable.